Is my prop the problem?

[1970Thunderbird]

Hey all,

I recently replaced the trigger, stator and internal harness on my 1978 Mercury 800. It's mounted on a 1970 Thunderbird Seminole. I was pleasantly surprised to discover I was getting close to 5300 rpm out of the motor at idle. I thought I was ready to really fly across the water.

Wrong.

A few facts for you:

*The boat is a cathedral trihull.
*It weighs about 1,000 pounds empty.
*With motor, fuel, batteries and people, it probably is close to 2,000lbs.
*The motor is a '78 Mercury 800, 80hp.

I had it out today, testing it out. Here's what I discovered.

*When I put the motor in forward, it will sometimes die. I have to baby the throttle to keep her running.
*I can't smoothly go to wide open. I have to baby it a bit and it tries to die on me, so I back it down. only after a few minutes of this can I go to wide open throttle, even if I've already been running for a while.
*At WOT in forward, the engine runs at 2,500 rpm, about half of what I expect. If I turn sharply, the prop starts cavitating and the motor revs much higher, up towards 5,000rpm.

I'm currently running a 48 72762A4 19p prop. I take it it has a 19 pitch? I don't know much about the prop. It was the prop that was on the motor when I purchased it used. My boat originally had a Chrysler outboard. I looked the prop on it and it has 13x14 stamped on it, which I take to mean it's a 13" diameter prop with a 14 pitch.

Truth be told, I'm a little lost. I'd try just using the prop from the Chrysler, but it won't fit. Do I have an engine problem, or am I just trying to push a prop that is too "aggressive" for such a heavy boat?

I'd like to pull a skier or tuber. Maybe two if I could. Also, the boat will be used for general cruising and fishing.

One more thing: I decided to have all three passengers wait on the shore while I tested something. With their weight out of the boat, it did move noticeably faster, but not full power, by any means. And I still had to jockey the throttle to get there.

Can you wonderful sages help me? If you think it's the prop, can you recommend a proper replacement? Or should I make my way back to my usual outboard troubleshooting forum?

Thanks a ton!

 

[Texasmark]

Re: Is my prop the problem?

5300 at idle? Not good for the engine. 1500-2000 is all you want to do.

Fresh fuel and TCWIII 2 cycle oil at the rate of 1 pt. per 6 gallons. Add a can of Sea Foam to clean up the engine. Sounds like engine is running lean which is not good for it either. Addressing the prop issue below will help it unload, thus not requiring as much fuel to do the job at hand, and should run a lot smoother and much better performance. The idle is adjustable at the engine and in gear in the water should idle at about 700 rpm. Pull the cowl, move the throttle linkage and look at it. You will see a spring loaded screw that limits the slow speed end of linkage travel. Screw it in some to increase that lowest speed setting.

Engine should top out at 5500 rpm for best engine performance at WOT with your normal load. Don't include your rpm's while towing a skier. That is over and above a normal load. You didn't give the length of your boat but the weight helps.

Your engine should be mounted on your boat (since you said that you changed engines) with the anti vent plate (the horizontal plate immediately above the prop) even with up to about 1" above the bottom of the boat at the transom. This should put it just at or slightly above the surface of the water at WOT and that is where you usually get the best performance. If you run it above it will surely take a cupped prop to stay locked in. As you slow down, obviously it will duck under and that's ok.

That engine probably sports a 2.3:1 gearbox and with that load you probably need to go down to a 17P or if you want to carry around a lot of people and ski, maybe a 15P. I would be inclined to run a 15P cupped. Turning Point makes a Hustler alum for sale on here at about $100 that is cupped and seems to be an upper cut alum from a performance standpoint. It has vent holes in it that will help you to pull your skier up faster with a boat load of folks. This lower pitch may (probably will)help your ventilating problem and being a cupped prop it surely will help it. I have one and for what and what it costs, it is a good prop.

A '78 engine is power head rated, not prop rated like they are today, which means your engine is really about a 70 hp and that is not a lot to do what you said you want to do. So, the only way to do that is to do what I said about the 15P prop and be happy with 25-27 mph top end....that's fast enough to have fun skiing.

Don't correlate the Chrysler with the Merc...different critters.

You have a lot of potential things to address, but we can take it nice and easy and walk through them as you do things and provide feedback. Getting the right prop is a good place to start.

HTH,

Mark

 

[1970Thunderbird]

Re: Is my prop the problem?

Mark,

Thanks for the reply. Here's what I know:

I've been running fresh fuel. 1 pint of Quicksilver TCW3 oil per 6 gallons of gas. I've added in some ethanol neutralizer. I'll add the seafoam next fill-up.

I think the engine is running a bit rich. It's not terribly smoky, but there is some smoke. I can't say for certain it's more than normal, though.

I've not yet tried to tow anything. All my results are from a boat with four people. Oh, and it's a 17 footer.

The motor is a long shaft, and the anti-vent plane is even with the keel. While running wide open yesterday, I looked and saw that the plate and whale-tale are either right above the surface or just below. Hard to say with all the spray, but it looked as though it was right on top of the water.

I know this boat will not do 80 miles an hour. I'm completely ok with that. What I would like to be able to do is carry 4-6 total people and tow a skier or tube. I'm fine with it not being a speed boat. Even if I can't tow, but still cruise the lake at a decent speed, I'll be happy. Do you think this is feasible with an 80hp motor? I should think so, as the original motor was a 70hp and it was used in the ocean. I'm wanting to carry my folks/friends out to enjoy the water. If skiing is impractical, that's ok, so long as cruising is enjoyable.

I'll definitely look into the prop. I do business with a company that sells marine and small engine supplies, so I'll see if they offer that prop. Also, I'm not looking to just throw money away, but wouldn't a stainless be better? And is three blades enough, or should I look for four? Or should I buy one or two fairly cheap ones, find which one works better, and then invest in a stainless of that one?

I only mention the Chrysler because it was the original motor. And it pushed the boat decently in the ocean, even with 70hp, so I imagine the prop had a fair amount to do with that.

I really appreciate the advice. I'll work on adjusting the idle speed. I have a test tank that's about 200 gallons. I should be able to engage the motor with the new prop in that. All else fails, I ought to be able to do that on the lake..

Thanks a ton!

 

[Texasmark]

Re: Is my prop the problem?

Ok, Thunderbird.

The name of the game is thrust. Thrust is prop pitch and rpm's to rotate that pitch under the most grueling of circumstances. It's that simple. It's called horsepower and the application thereof.

So the question is, how do you do that? Well it's not that simple as there is another part to the equation and that is.....well what about once you have overcome this grueling obstacle (the hole shot) and now you are up and planing, stresses are reduced and you want to click off some mph and you have your engines top rated rpms to consider. So that's the trick.

If you really aren't concerned with top end (and who's not, butttttt) you can pitch low and maybe get a vented prop, or go to a 4 blade for max hole shot with a boat load of folks and water toys in tow. Watch your rpm's, keep them within the mfgr's recommendations, regardless of speed of your boat and have fun......but what I just said is not completely true.

I have run Mercs since '74 and I have run them 500 or more rpm's above their rated limit and they LOVE IT. I have never lost an engine and have NEVER HAD AN ENGINE BACK TO A DEALER OR TO A TRAINED MECHANIC TO FIX A PROBLEM...7.5 HP, 85 HP, 115 HP, 90 HP current engine which by the way runs a 17' bass boat 50+ mph gps turning a 24P Ballistic SS 3 blade prop....the boat is light and fast.....not what you are referring to, but it is a point of reference.

HTH,

Mark

 

[1970Thunderbird]

Re: Is my prop the problem?

Mark,

Thanks for the reply. Since I last posted, I've run a compression check on the motor again, and basically found I'm due for a rebuild. Once I get it all back together, I'll check it with the current prop and see the results. A new prop may be in the future, but I've got to get her developing full power again.

Thanks, and I'll be back when I know more.

 

[Silvertip]

Re: Is my prop the problem?

Ethanol Neutralizer???? Where on earth do these terms come from. It is a "Fuel System Treatment" or "Fuel System Conditioner", or a "Fuel System Additive". If it was an Ethanol Neutralizer the engine wouldn't run because it would have likely neutralized the gasoline as well.

 

[Texasmark]

Re: Is my prop the problem?

Ethanol Neutralizer???? Where on earth do these terms come from. It is a "Fuel System Treatment" or "Fuel System Conditioner", or a "Fuel System Additive". If it was an Ethanol Neutralizer the engine wouldn't run because it would have likely neutralized the gasoline as well.

Ha! Had to laugh at this reply...............NGK's still running fine. Thanks, Mark

 

[steelespike]

Re: Is my prop the problem?

Well I would assume it is intended to reduce the negative effects of the ethanol not neutralize the ethanol.

 

[steelespike]

Re: Is my prop the problem?

Mark,

Thanks for the reply. Since I last posted, I've run a compression check on the motor again, and basically found I'm due for a rebuild. Once I get it all back together, I'll check it with the current prop and see the results. A new prop may be in the future, but I've got to get her developing full power again.

Thanks, and I'll be back when I know more.

If the motor seems reasonably sound a strong seafoam treatment to loosen up stuck rings.
It actually seems to work;just follow the directions on the container.